1. Field of the Invention
The invention relates to a developing apparatus in an image forming apparatus using an electrophotographic process such as a copying machine, a printer or a facsimile apparatus.
2. Related Background Art
In image forming apparatuses using, for example, an electrophotographic process, and above all, image forming apparatuses effecting color image formation, there has heretofore been widely utilized a two-component developing method using a mixture of a nonmagnetic toner (toner) and a magnetic carrier (carrier) as a developer. The two-component developing method, as compared with other developing methods presently proposed, has such merits as the stability of the quality of image and the durability of the apparatus.
In a developing apparatus using the two-component developing method (two-component developing apparatus), as the toner is supplied to an image bearing member to thereby effect development, toner density (the rates of the toner and the carrier, or the rate of the toner to the entire developer) in the developer is gradually reduced. To prevent this reduction in toner density, it is necessary to supply a fresh toner. When the toner is supplied, a two-component developer including also a carrier is sometimes supplied.
FIG. 12 of the accompanying drawings shows a conventional popular two-component developing apparatus. As prior art literature regarding such a two-component developing apparatus, mention may be made, for example, of Japanese Patent Application Laid-Open No. S55-32060.
The developing apparatus 201 has a developer container 202 containing a developer therein. A portion of the developer container 202 which is opposed to an image bearing member which is a member to be developed opens. A developing sleeve 211 as a developer carrying member for supplying the developer to the image bearing member is rotatably disposed in such a manner as to be partly exposed from this opening portion. The developer container 202 has a developing chamber (first developer containing chamber) 221 and an agitating chamber (second developer containing chamber) 222. The developing chamber 221 is disposed along the axial direction of the developing sleeve 211, and the developer contained therein is supplied to the developing sleeve 211. In the agitating chamber 222, a supplied toner and the developer in the developer container 202 are mixed together and agitated, and are conveyed in a direction opposite to the developing chamber 221. The developing chamber 221 and the agitating chamber 222 are comparted by a partition wall 225. However, the longitudinal opposite end portions of the partition wall 225 are formed with a first communicating path (connecting portion) 223 and a second communicating path (connecting portion) 224 permitting the passage of the developer. A first screw 213 and a second screw 214 as conveying members (developer conveying and agitating members) which effect the conveyance and agitation of the developer are disposed in the developing chamber 221 and the agitating chamber 222, respectively. Thereby, there is formed a developer circulating route (in a direction indicated by the arrow D in FIG. 12) in which the developer circulates between the developing chamber 221 and the agitating chamber 222, through the first communicating path 223 and the second communicating path 224.
A toner supplying port for supplying the toner is provided in the developer container 202. The toner supplied into the developer container 202 usually by the fall by gravity at a toner supplying position T is agitated with the developer in the developer container 202, in the agitating chamber 222. Thereby, the supplied toner contacts with a carrier and is frictionally charged.
However, when the agitation in the agitating chamber 222 is insufficient, the developer is conveyed from the first communicating path 223 to the developing chamber 221 while the supplied toner remains not sufficiently charged. If this insufficiently charged toner is used for the developing operation, toner fog may occur in an image blank portion (an area on the image bearing member to which the toner originally should not adhere), thus resulting in the lowering of the dignity of image.
In order to prevent such toner fog, in the conventional developing apparatus 201, it is ordinary to suitably set the distance from the toner supplying position T in the agitating chamber 222 to the first communicating path 223 (hereinafter referred to as the “agitating distance”) L1. In this case, however, the longitudinal distance of the developer circulating route (i.e., the length of the developing chamber 221 and the agitating chamber 222 in the axial direction (longitudinal direction) of the developing sleeve 211: hereinafter referred to also as the “longitudinal conveying distance”) L2 often becomes long. This becomes a factor which binders the downsizing of the developing apparatus 201, and further the downsizing of the entire image forming apparatus.
Due to the rise of needs for color images in recent years, also in image forming apparatuses for effecting color image formation, the downsizing of the apparatus has come to be required together with the lower cost of the apparatus.
The minimum longitudinal length required of the longitudinal conveying distance L2 in the agitating chamber 222 is nothing but the width of an electrostatic image formed on the image bearing member in a direction substantially along the longitudinal direction of the developer circulating route, i.e., a developing area (hereinafter referred to as the “image formable width”) A. That is, it will be good if the developer is supplied onto the developing sleeve 211 by a width corresponding to the image formable width A. Usually, the image formable width A substantially corresponds to an axial (longitudinal) length by which the developing sleeve 211 holds the developer thereon.
Now, Japanese Patent Application Laid-Open No. 2003-5519 discloses a construction as shown in FIGS. 13 and 14 of the accompanying drawings wherein a second screw 327 is provided while being inclined with respect to a first screw 326. In a developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, a developer falling space 334 for a developer conveyed by the first screw 326 to fall toward the second screw 327 is formed between the downstream side end portion of the first screw 326 with respect to a developer conveying direction and the upstream side end portion of the second screw 327 with respect to the developer conveying direction. Also, an inclined surface 335 for guiding the developer to the second screw 327 is provided below the developer falling space 334. The developer and the supplied toner fall onto the inclined surface 335 through the developer falling space 334. Then, the developer and the supplied toner are superposed upon each other and agitated, and slide on the inclined surface 335 and are conveyed to the upstream side end portion of the second screw 327 with respect to the developer conveying direction. Thereby, an attempt is made to improve the introducing performance of the supplied toner which will be described later.
However, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, in order to sufficiently charge the supplied toner from the second screw 327 to the first communicating path for delivering the developer to the first screw 326, the agitating distance L1 becomes longer than the image formable width A, and as a matter of course, the longitudinal conveying distance L2 in the agitating chamber also becomes longer than the image formable width A and thus, the developing apparatus becomes unnecessarily large.
Also, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, the supplied toner is made to fall into the developer falling space 334 formed in the lower portion of the first screw 327. Therefore, there is little or no developer near the first screw 326 right beneath the toner supplying port 336. As a result, it becomes impossible to stably supply the developer to the developing sleeve 325, and in this area, toner development cannot be effected. Accordingly, the longitudinal conveyed distance L2 in the agitating chamber becomes longer by at least a distance corresponding to the length of the developer falling space 334 than the image formable width A.
Further, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, the supplied toner falls onto the rotating first screw 326. Therefore, the supplied toner scatters in the upstream direction of the first screw 326 with respect to the developer conveying direction, and a back flow phenomenon is considered to occur. This back-flowing supplied toner is not sufficiently charged and therefore, if it is conveyed onto the developing sleeve 326, toner fog occurs to the image blank portion. Therefore, in the developing apparatus described in Japanese Patent Application Laid-Open No. 2003-5519, the longitudinal conveying distance L2 becomes longer than necessary, (than the image formable width A or more) by the length of the developer falling space 334, plus the distance of counter flow of the supplied toner.